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I have never run into something like this (and I work as a network administrator on my campus at school with a much more complicated network). Let me describe the scenario and what I've tried so far to no avail:
There are two computers; let's call them "Hackintosh" and "PC". I'm replacing PC with Hackintosh in one particular room of my house (which is wired for ethernet), and I've just finished configuring Hackintosh in another room where it was connected to a different ethernet jack perfectly fine (full network/internet access). This hackintosh's motherboard is a Gigabyte EP45-UD3R, which while slightly old runs OS X flawlessly. I wrote a really popular guide (one of the first) back in the day for how to install OS X 10.6 on that motherboard, and am really happy with how much advancement there has been. Anyways, here's what I did:
After configuring Hackintosh in a different room connected to that room's ethernet jack (successfully), I then tried connecting it to two other ethernet jacks in the house and it was able to get an IP automatically through DHCP instantly (aka the machine and its network card are operational and properly configured).
Before I disconnect PC in the room I'm moving Hackintosh into, PC's internet is working fine with the jack it has been connected to for over a year. I proceed to hook up Hackintosh to the same jack that PC was connected to, but OS X's network preference pane doesn't even register that it's connected to anything (it just says, "Cable Unplugged" etc). I tried two other cables that I know work (just in case) and that didn't change anything as I expected. Yet, Hackintosh is still able to obtain a DHCP IP using alternate jacks around the house.
I tried two alternate computers (a Macbook Pro and an old netbook running Windows) with the jack in question and they both obtained DHCP IPs instantly. It seems that Hackintosh is the only computer that cannot connect to my home network and obtain an IP using this specific jack.
Also, I reconnected PC to the jack in question it had been connected to and working with in the past, and it was able to obtain an IP. Just to be thorough I also tried connecting PC to other jacks around the house and it worked with each one.
I'm not sure what's left. I restarted my router (it's one of those AT&T U-Verse devices) and that still hasn't helped Hackintosh get a DHCP-assigned IP address from the jack PC was connected to. I figured there might be some weird issue where the router is filtering out Hackintosh's MAC address from using PC's port, but I don't know what would cause that. If no other computers besides PC worked using that jack I would assume the issue is some kind of automatic ISP/router mac filtering as PC has been the only computer connected to the jack in its room until I started moving stuff around today, but as I mentioned alternate Mac and Windows machines were able to get an IP from the jack successfully so I doubt that's the problem.
If it matters, before Hackintosh was Hackintosh, it was running Linux server software and connected to another jack in the house for a long time (one of the ones I tested when looking for alternate jacks).
Any ideas? I'm still thinking but probably about to sleep on it... As a super last resort I can clear the device list in my router and have it re-detect everything although given Hackintosh says, "Cable Unplugged" and not "Self-Assigned IP" or something I feel like that won't work.
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EDIT1: I did that "clear devices list" on my router and it hasn't changed anything. Hackintosh is still unable to receive a DHCP-assigned IP address specifically from that one jack I need it to. Rebooted and out of curiosity tried changing the "Ethernet Built-In" flag to "No" in the Chameleon boot plist, but it didn't change anything. I'm going to sleep but will check this thread in the morning and probably continue to tinker in hope of a solution.
EDIT2: Still have the problem (sleeping on it magically didn't make it work). I just noticed that the light in the ethernet port on the computer is blinking orange once every 5 or so seconds. I'm about to consult Google to see what that means...
EDIT3: The blinking orange light just means it's connected to a gigabit interface (as opposed to a green light which would mean it's connected to a 100 Mbps interface). I'm now going to try manually installing Lnx2Mac's Realtek RTL81XX driver using his .pkg file.
EDIT4: EUREKA! I'm typing this from Hackintosh using the questionable ethernet jack. The only catch? I'm actually connecting the questionable jack to a normal 10/100 ethernet switch (didn't have a gigabit one lying around) and then Hackintosh to another port on the same switch... I guess there must be something wrong with the physical wiring layout of the EP45-UD3R and/or the questionable jack. Can anyone give me an explanation? For the time being, I'm ok finding a small gigabit ethernet switch and using that as the "wiring proxy" to get Hackintosh to work in the room I need it to. I'll dismantle the jack later to see if I can find any anomalies.